1. Field of the Invention
The present invention is directed to a method for determining at least one parameter related to a patient's spontaneous attempts at inspiration and/or the patient's respiratory effort in spontaneous attempts at inspiration, as well as to an apparatus operating according to the method and making use of the parameter to assist breathing in the patient.
2. Description of the Prior Art
In normal, spontaneous inhalation, the respiratory musculature creates negative pressure in the chest cavity, causing air to be drawn down into the lungs. The negative pressure formed in the initial 100 ms of a breath is directly proportional to the respiratory incentive generated in the respiratory center of the medulla oblongata. The respiratory incentive reflects, in turn, the body's need for e.g. oxygen. Accordingly, a large respiratory incentive results in a deep breath, e.g. during heavy physical exertion when the body needs a large amount of oxygen. A normal value for the respiratory incentive is about a 2 cmH.sub.2 O drop in pressure in the first 100 ms. During expiration, the musculature relaxes, and air is expelled from the lungs.
In injuries and illness, a patient's ability to breathe may be so compromised that supplementary respiratory assistance must be provided by a breathing apparatus, usually a ventilator. This may also be the case when a patient's ability to breathe is suppressed, e.g. during anaesthesia.
The term "patient" as used herein refers, in principle, to all creatures which breathe with lungs, but to humans and domesticated animals in particular.
Ventilators are equipped with triggering systems, which induce an inspiration (inspiratory phase) whenever an attempt by the patient to inhale is detected. Triggering systems can be based on the measurement of pressure, the patient then being required to generate a drop in pressure sufficient to trigger the ventilator, or on the measurement of flow, the patient then being required to generate a gas flow sufficient to trigger the ventilator. The gas flow generated naturally depends on the negative pressure the patient is able to produce. Combinations of pressure measurement and flow measurement are also used.
The ventilator supplies breathing gas at the flow and pressure set by the physician for each patient. For example, the physician sets the apparatus pressure to which the patient is subjected at the end of an expiration (PEEP--Positive End Expiratory Pressure or, less commonly, NEEP--Negative End Expiratory Pressure). PEEP refers to a positive pressure in relation to the surroundings and can therefore range from 0 cmH.sub.2 O on up. The physician also selects the ratio between inspiratory duration and expiratory duration.
In other words, the patient must make some inspiratory effort in order to trigger the ventilator into delivering a flow of gas. If a patient is unable to make this effort, the ventilator must, in principle, exercise complete control over the patient's breathing. Even if this is essential to the survival of many patients, it could simultaneously contribute to a weakening or, at worst, atrophy of the patient's respiratory musculature. This leads, in turn, to prolonged recovery times and a heavier burden on treatment facilities (both in terms of costs and bed occupancy).
Effective triggering therefore could result in more rapid recovery and weaning off respiratory assistance for the patient. When a magnitude for the respiratory incentive is selected at which a large inspiratory incentive causes more breathing gas to be supplied, respiratory assistance can be better tailored to the patient's needs.
One problem in this context concerns the presence of residual positive pressure inside the lungs. In the present application, this residual positive pressure is designated Auto-PEEP. A number of factors can lead to the development of residual positive pressure in the lungs. Some of these factors are physiological, such as flow resistance in the lungs slowing evacuation of some parts of the lungs. Other factors are apparatus-related, such as the ratio between inspiration duration and expiration duration and between respiratory rate and tidal volume.
Auto-PEEP causes the true pressure gradient the patient needs to overcome in order to trigger the ventilator to exceed the value anticipated at the prevailing ventilator settings. The effort the patient must make increases, and the delivery of breathing gas is delayed.
Therefore, a more reliable way of determining when the patient starts an inspiration is needed so respiration can be facilitated in the best way possible.